Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Comparative ARPES Study on Iron-Platinum-Arsenide Superconductor Ca-10(Pt4As8)(Fe2-xPtxAs2)(5) (x=0.25 and 0.42) Reviewed

Masanori Sunagawa, Rikiya Yoshida, Toshihiko Ishiga, Koji Tsubota, Taihei Jabuchi, Junki Sonoyama, Satomi Kakiya, Daisuke Mitsuoka, Kazutaka Kudo, Minoru Nohara, Kanta Ono, Hiroshi Kumigashira, Tamio Oguchi, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 5 ) 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We studied the electronic structure of Ca-10(Pt4As8)(Fe2-xPtxAs2)(5) for two platinum concentrations with different T-c using ARPES. From the carrier number estimated from the areas of FS sheets, we found that the Pt substitution induces electron doping into the Fe2As2 layers (effectively two electrons per Pt atom). In addition, we found that the FS nesting condition in x = 0.25 is stronger than that in x = 0.42, indicating close correlation between the FS nesting and T-c.

DOI： 10.7566/JPSJ.84.055001

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Signature of high T-c above 25 K in high quality superconducting diamond Reviewed

Hiroyuki Okazaki, Takanori Wakita, Takayuki Muro, Tetsuya Nakamura, Yuji Muraoka, Takayoshi Yokoya, Shin-ichiro Kurihara, Hiroshi Kawarada, Tamio Oguchi, Yoshihiko Takano

APPLIED PHYSICS LETTERS 106 ( 5 ) 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have observed zero resistivity above 10K and an onset of resistivity reduction at 25.2K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower T-c. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest T-c diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high T-c diamond. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4907411

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Signature of high $T_c$ around 25K in higher quality heavily boron-doped diamond Reviewed

H. Okazaki, T. Wakita, T. Muro, T. Nakamura, Y. Muraoka, T. Yokoya, S. Kurihara, H. Kawarada, T. Oguchi, Y. Takano

Appl. Phys. Lett. 2015.2

16aDB-6 Angle-resolved photoemission spectroscopy on KxFe_<2-y>Se_2 II

Sunagawa M., Terashima K., Hamada K., Fujiwara H., Fukura T., Takeda A., Tanaka M., Takeya H., Takano Y., Arita M., Shimada K., Namatame H., Taniguchi M., Suzuki K., Usui H., Kuroki K., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1681 - 1681 2015

18pCH-1 Intrinsic electronic states of ferromagnetic metal CrO2 revealed by bulk-sensitive spin-resolved photoemission spectroscopy

Fujiwara H., Sunagawa M., Terashima K., Kittaka T., Ogata M., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 942 - 942 2015

16pDA-7 Relationship between Fermi surface topology and chemical pressure effect on Ln(O,F)BiS_2 studied by ARPES

Terashima K., Wakita T., Sunagawa M., Fujiwara H., Hamada T., Nagao M., Watauchi S., Tanaka I., Ono K., Kumigashira H., Okazaki H., Takano Y., Usui H., Kuroki K., Suzuki K., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1720 - 1720 2015

16aAD-4 Fe 3p-3d resonant photoemission study of mixed-valence compound YbFe_2O_4

Fukura T., Wakita T., Takeda A., Terashima K., Fujiwara K., Ikeda N., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 2405 - 2405 2015

17pCD-9 High-resolution photoelectron study on Ce(Os_<1-y>Re_y)_2Al_<10>

Nagayama T., Terashima K., Kawabata J., Yamada Y., Takabatake T., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1933 - 1933 2015

18aCE-9 高濃度ボロンドープダイヤモンド超伝導のB1s-2p共鳴光電子分光

竹田彩, 脇田高徳, 福良哲司, 高野義彦, 栗原慎一郎, 川原田洋, 小口多美夫, 村岡祐治, 横谷尚睦

日本物理学会講演概要集 70 1237 - 1237 2015

Proximity to Fermi-surface topological change in superconducting LaO0.54F0.46BiS2 Reviewed

Terashima Kensei, Sonoyama Junki, Wakita Takanori, Sunagawa Masanori, Ono Kanta, Kumigashira Hiroshi, Muro Takayuki, Nagao Masanori, Watauchi Satoshi, Tanaka Isao, Okazaki Hiroyuki, Takano Yoshihiko, Miura Osuke, Mizuguchi Yoshikazu, Usui Hidetomo, Suzuki Katsuhiro, Kuroki Kazuhiko, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW B 90 ( 22 ) 2014.12

Proximity to Fermi-surface topological change in superconducting LaO0.54F0.46BiS2 Reviewed

Kensei Terashima, Junki Sonoyama, Takanori Wakita, Masanori Sunagawa, Kanta Ono, Hiroshi Kumigashira, Takayuki Muro, Masanori Nagao, Satoshi Watauchi, Isao Tanaka, Hiroyuki Okazaki, Yoshihiko Takano, Osuke Miura, Yoshikazu Mizuguchi, Hidetomo Usui, Katsuhiro Suzuki, Kazuhiko Kuroki, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 90 ( 22 ) 2014.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The electronic structure of nearly optimally doped novel superconductor LaO1-xFxBiS2(x = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (E-F), which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near E-F shows a squarelike distribution around the Gamma(Z) point in addition to electronlike Fermi-surface (FS) sheets around the X(R) point, indicating that FS of LaO0.54F0.46BiS2 is in close proximity to the theoretically predicted topological change.

DOI： 10.1103/PhysRevB.90.220512

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Pressure-dependent magnetization and magnetoresistivity studies on tetragonal FeS (mackinawite): revealing its intrinsic metallic character Reviewed

S. J. Denholme, H. Okazaki, S. Demura, K. Deguchi, M. Fujioka, T. Yamaguchi, H. Takeya, M. ElMassalami, H. Fujiwara, T. Wakita, T. Yokoya, Y. Takano

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 15 ( 5 ) 2014.10

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

The transport and magnetic properties of the tetragonal Fe1+delta S were investigated using magnetoresistivity and magnetization within 2 <= T <= 300 K, H <= 70 kOe and P <= 3.0 GPa. In addition, room-temperature x-ray diffraction and photoelectron spectroscopy were also applied. In contrast to previously reported nonmetallic character, Fe1+delta S is intrinsically metallic but due to a presence of a weak localization such metallic character is not exhibited below room temperature. An applied pressure reduces strongly this additional resistive contribution and as such enhances the temperature range of the metallic character which, for similar to 3 GPa, is evident down to 75 K. The absence of superconductivity as well as the mechanism behind the weak localization will be discussed.

DOI： 10.1088/1468-6996/15/5/055007

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Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies Reviewed

Toshihiko Ishiga, Takanori Wakita, Rikiya Yoshida, Hiroyuki Okazaki, Koji Tsubota, Masanori Sunagawa, Kanta Uenaka, Kozo Okada, Hiroshi Kumigashira, Masaharu Oshima, Keisuke Yutani, Yuji Muro, Toshiro Takabatake, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 9 ) 2014.9

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (E-F) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that E-F is located at the midpoint of the gap.

DOI： 10.7566/JPSJ.83.094717

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Persistent insulator-to-metal transition of a VO2 thin film induced by soft X-ray irradiation Reviewed

Yuji Muraoka, Hiroki Nagao, Shinsuke Katayama, Takanori Wakita, Masaaki Hirai, Takayoshi Yokoya, Hiroshi Kumigashira, Masaharu Oshima

JAPANESE JOURNAL OF APPLIED PHYSICS 53 ( 5 ) 2014.5

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We present a persistent soft-X-ray-induced insulator-to-metal transition at room temperature for VO2 thin films. Continuous soft X-ray irradiation renders the insulating electronic state metallic in the VO2 thin films, which has been found by photoemission spectroscopy measurement. The observed transition is irreversible and the metallic state persists when soft X-ray irradiation is stopped. The analysis of valence band spectra reveals that the density of states (DOS) of the V 3d band increases with irradiation time, while the DOS of the O 2p band decreases. We propose a simple model where the persistent insulator-to-metal transition is driven by oxygen desorption from VO2 thin films under soft X-ray irradiation. The present results will help in developing a new route for controlling the phase transition of VO2 by light. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB09

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Characteristic two-dimensional Fermi surface topology of high-T-c iron-based superconductors Reviewed

Masanori Sunagawa, Toshihiko Ishiga, Koji Tsubota, Taihei Jabuchi, Junki Sonoyama, Keita Iba, Kazutaka Kudo, Minoru Nohara, Kanta Ono, Hiroshi Kumigashira, Tomohiro Matsushita, Masashi Arita, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

SCIENTIFIC REPORTS 4 2014.3

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-T-c). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-T-c materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with T-c = 45 K, which is the highest T-c found for the AEFe(2)As(2) bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole-and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole-and electron-doped 122-type high-T-c superconductors.

DOI： 10.1038/srep04381

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Soft X-ray Photoemission Study of New BiS2-Layered Superconductor LaO1-xFxBiS2 Reviewed

Shinsuke Nagira, Junki Sonoyama, Takanori Wakita, Masanori Sunagawa, Yudai Izumi, Takayuki Muro, Hiroshi Kumigashira, Masaharu Oshima, Keita Deguchi, Hiroyuki Okazaki, Yoshihiko Takano, Osuke Miura, Yoshikazu Mizuguchi, Katsuhiro Suzuki, Hidetomo Usui, Kazuhiko Kuroki, Kozo Okada, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 3 ) 2014.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We use core level and valence band soft X-ray photoemission spectroscopy (SXPES) to investigate the electronic structure of new BiS2 layered superconductor LaO1-xFxBiS2. The core level spectra of doped samples show a new spectral feature on the lower binding energy side of the Bi 4 f main peak, which may be explained by core-hole screening with metallic states near the Fermi level (E-F). The experimental electronic structure and its x dependence (a higher binding energy shift of the valence band as well as appearance of new states near E-F having the dominant Bi 6p character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near EF from that of calculations might provide insight into interesting physical properties. This study provides the first experimental electronic structure of the new BiS2 layered superconductors.

DOI： 10.7566/JPSJ.83.033703

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Evidence for metallic states in potassium-intercalated picene film on graphite Reviewed

Okazaki Hiroyuki, Jabuchi Taihei, Wakita Takanori, Kato Takashi, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW B 88 ( 24 ) 2013.12

Evidence for metallic states in potassium-intercalated picene film on graphite Reviewed

Hiroyuki Okazaki, Taihei Jabuchi, Takanori Wakita, Takashi Kato, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 88 ( 24 ) 2013.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We report the results of photoemission spectroscopy on the potassium concentration (x) dependence of K-x picene films on graphite. The results show a systematic shift of a whole valence band and the appearance of a clear Fermi edge for a certain value of x, with additional structures at higher binding energies. Observed spectral evolutions are reminiscent to those of doped C-60, implying that the K-x picene film has multiple phases depending on the K concentration. Appearance of a clear Fermi edge, in contrast to multilayer K-x picene films on Au, SiO2, and Ag(111) substrates, where no clear Fermi edges are observed, suggests that the emergence of the metallic states for K-x picene films depends on the specific molecular orientations in the films on different substrates.

DOI： 10.1103/PhysRevB.88.245414

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Collapsed Tetragonal Phase Transition of Ca(Fe1-xRhx)(2)As-2 Studied by Photoemission Spectroscopy Reviewed

Koji Tsubota, Takanori Wakita, Hiroki Nagao, Chiaki Hiramatsu, Toshihiko Ishiga, Masanori Sunagawa, Kanta Ono, Hiroshi Kumigashira, Masataka Danura, Kazutaka Kudo, Minoru Nohara, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 ( 7 ) 2013.7

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have investigated the changes in the band structure and core-level energy of Ca(Fe1-xRhx)(2)As-2 across the tetragonal-collapsed tetragonal (T-cT) phase transition using photoemission spectroscopy. Angle-resolved photoemission spectroscopy clearly demonstrates that, at the Gamma point in the Brillouin zone, a hole-like band that crosses the Fermi level in the T phase lowers its energy position and is completely filled in the cT phase. This indicates the disappearance of a hole-like Fermi surface around the zone center in the cT phase, which gives rise to the extinction of nesting. The relationship of the present results with the results of previous transport and magnetic studies is discussed. The temperature dependences of the As 3d and Ca 2p core levels show energy shifts toward higher and lower binding energies, respectively. These data constitute the first direct experimental results on electronic structure and chemical states across the T-cT transition.

DOI： 10.7566/JPSJ.82.073705

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Magnetoelectric Effect Driven by Magnetic Domain Modification in LuFe2O4 Reviewed

Takashi Kambe, Yukimasa Fukada, Jun Kano, Tomoko Nagata, Hiroyuki Okazaki, Takayoshi Yokoya, Shuichi Wakimoto, Kazuhisa Kakurai, Naoshi Ikeda

PHYSICAL REVIEW LETTERS 110 ( 11 ) 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T-Ferri similar to 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4. DOI: 10.1103/PhysRevLett.110.117602

DOI： 10.1103/PhysRevLett.110.117602

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Papers - YOKOYA Takayoshi